The vast majority of all engine-driven vehicles in operation today use internal combustion engines using either a Diesel cycle or the Otto cycle. A very few automotive vehicles are powered by external combustion engines such as gas turbines, Stirling cycle engines, or steam engines. A relatively small number of vehicles are powered by electric motors.
Each class of motor vehicle propulsion system has its benefits and detriments. The Diesel cycle engines are simple and robust while utilizing significant amount of the energy found in its hydrocarbon fuel. The exhaust from most such diesel cycle engines is high in nitrogen oxides and carbon particulates. The Otto cycle engines are probably the most highly engineered mechanical device existing on earth. Although the efficiency of Otto cycle power plants as used in automotive vehicles has significantly improved since their first use in the latter part of the 19th century, their efficiency (based on the potential energy content of the fuel) is not high because of the low compression ratios associated with gasoline engines. In general, these engines can be made into quite lightweight packages for use in a variety of vehicles.
Vehicles using electric motors are currently not as flexible and practical as are those using one of the internal combustion engine power plants. Although acceleration and top speed of such electric vehicles may match those of internal combustion-engined vehicles, electric motor-powered vehicles have a significant detriment because of their need for batteries. A variety of different battery systems have been proposed for use in such vehicles. Lead-sulfuric acid batteries remains the primary choice for such vehicles as other, more advanced technology, batteries are being developed. Obviously, lead-acid batteries are quite heavy and often have a lengthy charging cycle. Such cars have a short vehicle range. Unlike the internal combustion-engined vehicles, those powered with electric motors have few if any vehicular emissions. Obviously though, the utility power plants which provide electric power to battery-powered electric cars will be responsible for some type of emission.
The motive engines discussed above are traditionally used for vehicular power. More recent approaches include hybrid electric and fuel-cell power systems. Hybrid electric systems generally make use of internal combustion units to recharge batteries. Fuel-cell systems often require very large hydrogen storage vessels. An alternative to hydrogen storage systems is the use of a continuous partial oxidation or steam reforming on board the vehicle. Our invention is an alternative to these.
Steam Reforming
Petroleum fuels may be reformed using steam to yield hydrogen. The procedure requires two moles of water and heat to decompose thermally the hydrocarbon according to the following reaction: EQU CH.sub.x +2H.sub.2 O.fwdarw.CO.sub.2 +(x/2+2)H.sub.2
Using methane as a feedstock, x=4 and the theoretical hydrogen yield is four moles of hydrogen for each mole of methane fed to a reactor. The reaction is endothermic so that the theoretical heat required is 61.8 Kcal/gm-mole, the heat of combustion of methane is 191 Kcal/gm-mole, and the hydrogen efficiency is 91%. At equilibrium conditions, only about 2.55 moles of hydrogen is produced and therefore the hydrogen efficiency is reduced to 76%.
For a typical liquid petroleum fuel such as heptane, x=2.29, the theoretical hydrogen yield is 3.15 moles of hydrogen per mole of heptane, the heat required is 53.8 Kcal/gm-mole, the heat of combustion of heptane is 153 Kcal/gm-mole, and the hydrogen efficiency is 88%. However, at equilibrium conditions, the hydrogen production is reduced to 2.6 moles, and the corresponding hydrogen efficiency is reduced to 74%. This approach results in the highest hydrogen efficiency since some of the hydrogen is supplied by the steam which is broken down in the reforming reaction.
Partial Oxidation
In the partial oxidation of petroleum fuels, a portion of the fuel is burned to provide heat to decompose the fuel and water in an oxygen-starved environment, thus: EQU CH.sub.x +(1.5-x/8)H.sub.2 O+1/4(1+x/4)H.sub.2.fwdarw.CO.sub.2 +(1.5+3x/8)H.sub.2 +heat
For methane: EQU CH.sub.4 +H.sub.2 O+1/2O.sub.2.fwdarw.CO.sub.2 +3H.sub.2 +29.48 Kcal/gm-mole
The theoretical hydrogen efficiency for methane is 78% and at equilibrium conditions expected hydrogen efficiency at 64%. Using heptane as a feedstock, x=2.29 and the resulting equation is: EQU CH.sub.2.29 +1.21H.sub.2 O+0.39O.sub.2.fwdarw.CO.sub.2 +2.36H.sub.2 +25.1 Kcal/gm-mole
The theoretical hydrogen efficiency is 77% and at equilibrium the hydrogen efficiency falls to 60%.
Partial oxidation has a moderately high hydrogen efficiency since some portion of the hydrogen contained in the feedstock is combusted in this reaction scheme.
Pyrolysis
Pyrolysis is the direct thermal decomposition of petroleum according to the following equation: EQU CH.sub.x +heat.fwdarw.C+x/2H.sub.2
For methane it is simply: EQU CH.sub.4 +heat.fwdarw.C+2H.sub.2
The heat required to decompose methane is about 18.9 Kcal/gm-mole and the corresponding theoretical hydrogen efficiency is 55%. Under equilibrium conditions, one could expect to extract about 90% of the hydrogen contained in the methane and the hydrogen efficiency is therefore reduced to 50%.
Using heptane, the reaction becomes the following: EQU CH.sub.2.29 +heat.fwdarw.C+1.14H.sub.2
The heat required in this reaction is 18 Kcal/gm-mole and the theoretical efficiency based on hydrogen is 40%. Experiments have shown that 90% of hydrogen can be recovered, thus reducing the overall hydrogen efficiency to 36%.
The pyrolysis process has the lowest hydrogen efficiency since only the hydrogen contained in hydrocarbon feedstock is available. It, however, has the advantage of being free of carbon monoxide and carbon dioxide gases which require further processing before utilization by fuel cells. Further, carbon monoxide is a reactive pollutant which must be combusted in an oxygen-rich atmosphere to meet the requirements for various transportation applications.
With the background stated above: U.S. Pat. No. 4,070,993 (to Chen) describes a process for cracking a low octane fuel to produce a gaseous product of substantially higher octane value which is then fed directly to an internal combustion engine. The partially hydrogenated gas generally contains gases of C.sub.5.sup.- for use in the engine. Conversion is shown in the examples to be 60% or greater. The remainder is presumably coke on the catalyst.
U.S. Pat. No. 4,862,836 (to Chen et al.) shows a similar process but one involving a dual converter involving a partial combustion of a fuel in the presence of oxygen. This partial combustion is necessary to sustain the heat required by the conversion, which conversion is apparently a partial oxidation or other similar reaction.
Other systems for reforming fuel prior to its combustion in an internal combustion engine are shown in variety of patents. For instance, U.S. Pat. No. 4,143,620 (Noguchi et al.) shows a fuel reforming system in which the catalyst used to reform the fuel is placed in heat exchange with the exhaust gas emanating from the engine. The fuel is preferably methanol. The methanol is partially sent to a carburetor where it is ignited in conjunction with a hydrogen-rich fuel stream which is produced by reforming the remainder of the methanol.
U.S. Pat. No. 4,147,136 (to Noguchi et al.) show a similar process but one in which the feedstock is a hydrocarbon fuel. The process utilizes a fuel reforming system which burns a portion of its hydrocarbon fuelstock to maintain a reforming reaction vessel at an appropriately high temperature. The remainder of the fuel is fed to that reforming reactor vessel. The resulting reformed gaseous mixture contains a substantial amount of hydrogen. The inclusion of hydrogen in the reformed gaseous mixture is said to facilitate a reliable ignition and combustion of that reformed mixture of the reformed mixture along with a non-reformed hydrocarbon fuel. This process allows the use of a lean air-to-fuel ratio which is further said to result in the lowering of HC, CO, and NO.sub.x emissions.
U.S. Pat. No. 4,147,142 (to Little et al.) describes a procedure for both modifying the physical state and chemical composition of a fuel prior to its combustion. The modification step includes vaporization of the liquid fuel and use of the heat from the engine's exhaust also to thermally crack the fuel passing to the combustion process.
U.S. Pat. No. 4,185,966 (to Frie et al.) shows a device for reforming a vaporized or atomized, liquid, higher hydrocarbon stream with an oxygen containing gas at an elevated temperature to form a gas mixture containing methane, carbon monoxide, and hydrogen. The product of the reformed gas generator is mixed with an exhaust stream and fed to an internal combustion engine.
U.S. Pat. No. 4,722,303 (to Leonhard) shows a method for using the heat of the exhaust gas to decompose methanol feedstock into hydrogen and carbon monoxide. This decomposed gas is injected directly into an internal combustion engine and, it is said, that by doing so towards the end of the compression cycle, an overall efficiency increase of 30% (as compared to a Diesel cycle engine) may be obtained.
U.S. Pat. No. 4,735,186 (to Parsons) also describes the procedure for cracking hydrocarbon fuel and passing it, along with a portion of the exhaust gas produced by combustion, along to the combustion chamber in an internal combustion engine.
U.S. Pat. No. 5,343,699 (to McAlister) describes using waste heat rejected from the heat engine to aid in generating a carbon monoxide and hydrogen rich fuel for a heat engine. The hydrocarbon fuel is combined with an oxygen donor to produce a fuel with a thermal value that is greater than the original fuel.
None of these procedures show partial or total pyrolysis of a hydrocarbon fuel, a separate conversion of the hydrogen stream formed by that pyrolysis in a fuel cell to produce electrical energy, and the simultaneous conversion of carbon coke to a mixture of carbon monoxide and hydrogen for use in a combustion engine.
Other procedures are known for the production of hydrogen which is further used as fuel in an internal combustion engine.
U.S. Pat. No. 4,362,137 (to O'Hare) shows an alternative fuel mixture, ignition, and an induction system in which an increased burning rate is achieved by directing conventional fuel through a pyrolysis cell and a cooler on its way to the combustion chamber. During such a passage, the fuel is partially stripped of its hydrogen and the so-stripped fuel is passed into the cell during the compression cycle.
A somewhat exotic process is shown in U.S. Pat. No. 4,597,363 (to Emelock) in which oxalic acid, dispersed glycerol, and formic acid is heated at a higher temperature to form hydrogen. The hydrogen is used directly in the fuel cell to produce electricity or as fuel for an internal combustion engine.
Again, none of these procedures show the central concept of using the partial or total pyrolysis of a hydrocarbon fuel to produce a hydrogen stream which is used in an electrical fuel cell to produce electricity and to use the remaining carbon coke to produce carbon monoxide and hydrogen fuel in an internal or external combustion engine.